This invention relates generally to apparatus for mounting and fastening printed circuit boards or cards within a rack or chassis, and, more particularly, to apparatus of the kind that includes an elongated wedge assembly that expands transversely to fasten the board in place.
Elongated wedge assemblies of this particular kind have been in common use for fastening printed circuit (i.e., PC) boards within an elongated slot in a rack or chassis. The assembly typically includes a center wedge having sloped surfaces at its opposite ends and two end wedges having surfaces that abut against the center wedge's opposite ends. A screw extends lengthwise through the center wedge and connects together the two end wedges. Rotating the screw in one direction draws the two end wedges toward each other, causing them to deflect transversely on the sloped abutting surfaces. This increases the assembly's effective width and thereby fastens the PC board tightly within the slot. Subsequently rotating the screw in the opposite direction moves the two end wedges apart from each other, to bring them back into longitudinal alignment with the center wedge and, thereby, to release the clamping force on the PC board. The wedge assemblies can be constructed with any uneven number of wedge segments.
Although the wedge assembly described briefly above has proven generally effective in locking a PC board in place, its use has not proven to be entirely satisfactory. This arises because some applications require that a predetermined force be applied to the PC board, to ensure contact between the board and the chassis track and, thereby, to provide efficient thermal conductivity and/or electrical grounding. A predetermined force is also sometimes required to prevent loosening due to critical vibration forces. In all of these cases, torque wrench measurements have normally been required to provide the desired clamping forces. Further, wedge assemblies of this kind can sometimes damage the PC board, because of overtightening of the screw.
Still another drawback to conventional locking wedge assemblies arises when the screw is loosened, which increases the assembly's length and allows the three wedges to move with respect to each other. Consequently, rotation or transverse movement of the wedges about the screw axis can sometimes make installation of the attached assembly and PC board to the chassis track difficult.
It should, therefore, be appreciated that there is a need for a locking wedge assembly that avoids the above difficulties. The assembly should ideally control the force applied to the PC board and chassis track, and thereby eliminate the need to measure torque each time the assembly is tightened. The wedge assembly also should maintain the wedges aligned with each other when the clamping force is removed, for insertion or extraction of the PC board to or from a chassis track. These features are all provided by the present invention.